1. Field of the Invention
The present invention relates to a non-contact, non-destructive measurement method of measuring film thicknesses of a multilayered sample object in which one or two transparent films are disposed on an SOI substrate which is formed by a body, a transparent insulation film and a monocrystalline or polycrystalline silicon film.
2. Description of the Background Art
In recent years, fabrication of an LSI on an SOI substrate has been becoming popular. FIGS. 15 and 16 are cross sectional views each showing an SOI substrate which serves as background of the present invention. In FIGS. 15 and 16, a silicon oxide film (transparent insulation film) 1 is formed on a silicon body B and a monocrystalline silicon film 2 is formed on the silicon oxide film 1, thereby constituting an SOI substrate 10. As compared with a conventional LSI (which is build on a bulk semiconductor substrate), an LSI built on the SOI substrate 10 has better device characteristics.
However, as fabrication of an LSI on an SOI substrate requires increasingly more complex, control of film thicknesses needs be more accurate than never. In some cases, thicknesses d1 and d2 of the silicon oxide film 1 and the silicon film 2 which are formed on the silicon body B need be measured. In other cases where one or two transparent films which are to be formed on the SOI substrate 10 during fabrication of an LSI, it is necessary to measure thicknesses d3 and d4 of these transparent films during the fabrication (For example, a silicon oxide film 3 needs be formed in FIG. 15 and a silicon oxide film 3 and a silicon nitride film 4 need be formed FIG. 16). Further, the need for measurement of the thicknesses d1 to d4 at the same time is mounting. Of course, measurement of the thicknesses d1 to d4 must be non-contact and non-destructive since the thicknesses must be measured during fabricating of an LSI.
Despite such needs, non-contact and non-destructive measurement of the respective thicknesses of the multilayered samples of FIGS. 15 and 16 is difficult. In reality, there has been no choice but to measure the thicknesses by destructing the multilayered sample object and observing the destructed sample object with an electron microscope or etc.
To improve the situation, techniques for measuring the thickness of each layer of a multilayered sample object have been proposed as that disclosed by U.S. Pat. No. 4,999,509. According to the disclosed technique, a film thickness range of each layer is inputted in advance and the thicknesses of the respective layers are measured using a global optimization method and a local optimization method.
The measurement of film thicknesses according to the U.S. Pat. No. 4,999,509, however, is not convenient for an operator since the operator must input a film thickness range of each layer in advance to perform the global optimization method. Although this disadvantage can be overcome by setting the film thickness ranges wide enough, expansion of the ranges leads to an increase in the number of computation steps, which in turn considerably extends a computation time. In addition, since a value calculated as a result of optimization largely varies depending on a starting point of optimization (i.e., the value of a film thickness of each layer) and other optimization parameters, a reproduction accuracy of measurement greatly drops depending on setting conditions.